This invention relates to a mechanism for driving a screw rod to move a work rack in an axial direction of the screw rod by making the screw rod rotate.
First, FIG. 8 shows a prior art supersonic motor comprising a rotor R and two vibrators S1 and S2. The vibrator S1 is made to vibrate so that the rotor R is pushed by the vibrator S1 to rotate in a clockwise direction in the figure. The vibrator S2 is made to vibrate so that the rotor R is pushed by the vibrator S2 to rotate in a counterclockwise direction in the figure.
Secondly, FIG. 9 shows a prior art mechanism for driving a screw rod by using a motor to rotate a helical rod.
The mechanism shown in FIG. 9 is constituted of a pair of stands 23, a helical rod 22 rotatably held between the pair of the stands 23 via bearings 24, a nut 25 movable in an axial direction of the helical rod 22 upon rotation of the helical rod 22, and a servo-motor 28 connected with the helical rod 22 via a coupling 27 for rotating the helical rod 22.
The helical rod 22 is provided with a helical groove 29 formed thereabout along the axial direction thereof, and balls 26 enclosed in the nut 25 are engaged in the groove 29 to hold the nut 25 on the helical rod 22. Thus, the servo-motor 28 makes the helical rod 22 rotate forward or in reverse so that the nut 25 may be moved in the axial direction of the helical rod.
A workpiece moved upon movement of the nut 25 is slidably held by rails, a slider or the like (not shown in FIG. 9), so that the load of the workpiece is not moved in a radial direction of the helical rod 22.
However, the above-mentioned mechanism for driving a helical rod has the following problems: troubles by back-lash occurs by a minimum space necessary in an engaged portion between the helical rod 22 and balls 26; space-savings may not be achieved because the balls, the nut, the servo-motor, and so on require sufficient space to rotate smoothly; back-lash is increased due to the increase of abrasion between the nut, the balls and the helical rod after a long period of use; and friction between the nut, the balls and the helical rod is increased by minimizing the space between them in order to decrease the back-lash in the mechanism.
Note that the helical rod is the same shape as a screw rod of the present invention, but the helical rod is used with the balls and the nut. In this point, the helical rod is distinguished from the screw rod of the present invention.